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Sandstorm over Gao, Mali, during the drought of 1984/1985.Credit: frans lemmens/Alamy Stock Photo.

Artificially cooling the planet through solar geoengineering could have some dramatic side effects – including an increase in droughts and hurricanes in some regions – if it is carried out in an unregulated way, a new study warns.

The study focuses on one proposed type of geoengineering, known as a “stratospheric aerosol injection”, which involves sending up substances to the stratosphere that are known to have a cooling effect on the climate.

It finds that if only one country, region or hemisphere were to pursue this type of geoengineering, other parts of the world could face adverse consequences. For example, if only the northern hemisphere were to release aerosols, the Sahel in Africa and parts of India would have to cope with more droughts.

The research highlights that “geoengineering is going to require more global cooperation than has ever been attempted before”, another scientist tells Carbon Brief.

Mimicking volcanoes

Solar geoengineering, or “solar radiation management” (SRM), describes an array of methods – all of which remain hypothetical – for artificially reducing sunlight at the Earth’s surface in order to dampen global warming.

It’s worth noting that while these proposed technologies may potentially be able to cool the planet, but they would not reduce the amount of CO2 in the atmosphere. SRM couldn’t, therefore, address all the impacts of climate change, including ocean acidification.

The new study, published in Nature Communications, investigates the potential physical side effects that could come about from artificially introducing aerosols into the atmosphere.

Research into the impact of major volcanic eruptions on global temperatures has shown that releasing certain aerosols into the stratosphere has an overall cooling effect on the planet. When a volcano erupts, it sends a cloud of ash high into the atmosphere. The sulphur dioxide released in the plume combines with water to form sulfuric acid aerosols, which can reflect sunlight.

Researchers have proposed that artificially introducing aerosols into the atmosphere, via a plane or a balloon, could have similar cooling effect.

A geoengineered world

Releasing aerosols from just one part of the world is likely to cause unwanted effects elsewhere, the new study finds. It uses modelling to simulate how an annual atmospheric injection of sulphur dioxide from both the northern and southern hemisphere could affect the climate between 2020 and 2070.

If aerosols were released just from the northern hemisphere, other parts of the world could face an increase in droughts, hurricanes and storms, says lead author Dr Anthony Jones, an atmospheric scientist at the UK’s Met Office. He tells Carbon Brief:

“If solar geoengineering were to be deployed solely in the northern hemisphere, then the resultant changes would reduce precipitation in the Sahel, and other regions such as India, and reduce the number of storms in the North Atlantic basin.

“To put it short, northern hemisphere solar geoengineering would be good for the southeast US, the Caribbean, and Mexico in terms of dissipating storms, but be very bad for the Sahel. In contrast, solar geoengineering in the south would enhance precipitation in the Sahel, but would also enhance the number of storms in the North Atlantic.”

Cyclone frequency

The focus of the paper is on how SRM could affect the formation of tropical cyclones. These are large storms that develop over tropical waters. They go by different names depending on where they develop – they are called hurricanes in the north Atlantic and northeast Pacific, typhoons in the northwest Pacific, and cyclones in the Indian Ocean.

The differing effects of SRM on cyclones come about as a result of how the location of an aerosol injection affects the tropical jet stream – a thin, fast flowing ribbon of air high up in the atmosphere. Jones explains:

“North Atlantic storm activity is related to the position of the tropical jet stream. If the jet coincides with Atlantic hurricane main development region (MDR) then the enhanced vertical wind shear acts to weaken storms. Solar geoengineering in the north would shift the jet stream south so that it coincides with the MDR, while solar geoengineering in the south would shift the jet stream north providing optimal conditions for storm formation.”

The graph below shows the frequency of tropical cyclones across the world between 1950 and 2000, and how this could change under a range of future scenarios between 2020 and 2070.

The dark blue line shows a scenario with an annual aerosol injection in the southern hemisphere, while red shows a scenario with an injection in the northern hemisphere. The graph also includes a climate scenario with no geoengineering (purple) and a scenario with an injection in both hemispheres (turquoise).

Tropical cyclone frequency from 1950-200 and under a range of simulated scenarios from 2020-2070, including an annual aerosol injection in the northern hemisphere (blue), an annual aerosol injection in the southern hemisphere (red), a global aerosol injection (turquoise) and a climate scenario with no geoengineering (purple). Black lines indicate the start and end of geoengineering. Source: Jones et al. (2017)

The graph appears to show that an aerosol injection carried out in both hemispheres would have little impact on the overall rate of tropical cyclones. However, it is not possible to tell from the simulations if this is truly the case, Jones explains:

“Our results are less conclusive for a symmetrical scenario than for the hemispheric scenarios. In particular, we highlight a long-standing disparity between explicitly modelled storms and statistical-downscaling techniques.”

The two types of models throw up different results. Global climate models suggest that tropical storms will fall in the coming decades as a result of continued clean air regulation, while regional climate models suggest that sea surface warming will lead to an overall increase in storms. Jones adds:

“It is important that we identify the reasons why the two storm tracking methods differ before we can conclusively assess the storm changes under a symmetrical solar geoengineering scenario.”

“One aspect of this is that the study goes beyond some of the usual things that we look at, like temperature and precipitation, and it’s aimed at extreme events, in this case hurricanes. These are the things that impact people’s lives directly and they need to be studied.”

However, there still a need for more research to understand all the potential impacts of SRM, he adds:

“I would really like to see this study replicated by other models. One reason is that these results are from one model. The authors did a great job of understanding why the model behaves the way it does, and they tie those behaviours to physical mechanisms, so the results are definitely plausible. But we need to understand whether the results are different for a different model, and if they are, why.”

Controlling the future

While SRM technologies remain in the realm of theory, interest in their possible development is continuing to grow. In October of this year, Carbon Brief attended a major geoengineering conference held in Berlin, where scientists, policymakers and ethicists debated whether or not the world should invest in SRM.

And earlier this month, the US Committee on Science, Space and Technology held a meeting to discuss geoengineering, with proposed SRM technologies – including cloud brightening – dominating the conversation.

The new findings should prompt world leaders to think about the challenges of governing solar geoengineering on an international scale, Jones says:

“My concern is that the positive impacts for one region may motivate an individual to deploy solar geoengineering in a single hemisphere at the expense of severe negative impacts to another region.

“Stratospheric aerosol injection would be relatively inexpensive and there is very limited if any existing governance that could deter individual actors or that could be used to hold actors accountable for any harmful consequences.”

“I don’t see it has any particular significance for the [geoengineering] debate. It was already obvious that doing solar geoengineering on just one side of the equator would not make sense.”

But Janos Pasztor, executive director of the Carnegie Climate Geoengineering Governance Initiative (C2G2) and former assistant UN secretary-general, echoed Jones’s call for more discussions into the global governance of solar geoengineering. He tells Carbon Brief:

“If one country decided to put its own interests first – say the leader of that country thought ‘our country needs cooling down, let’s do some regional solar geoengineering’ – that could have potentially catastrophic effects in other parts of the world.”

Ensuring that “all people and all parties” are represented in decisions involving geoengineering will be a complex challenge, he adds:

“Geoengineering is going to require more global cooperation than has ever been attempted before. What is clear is that a geoengineered world will have winners and losers, both geographically but also as we look towards future generations, because some generations will benefit more than others.”